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Stability and control characteristics of model helicopters

Visagie, Jonathan Gerhardus (2004-12)

Thesis (MScEng)--University of Stellenbosch, 2004.

Thesis

ENGLISH ABSTRACT: A need exists for the development of an unmanned rotorcraft capable
of autonomous flight, as would be required for the survey of high
voltage electricity supply lines. A program was initiated at the
University of Stellenbosch in December 2002 in order to develop such
an aircraft.
The first goal of this thesis was the development of software that could
calculate the stability and control derivatives of a model helicopter.
These derivatives could then be used in the formulation of an
appropriate helicopter control strategy. The second goal of the thesis
was an investigation of the stability and control characteristics of model
helicopters.
The trim settings of the helicopter were required in the calculation of
the stability and control derivatives. A computer program was
developed to determine the trim settings of a helicopter in forward
flight. Another program was developed to calculate the stability and
control derivatives, using the results of the trim analysis.
The trim analysis was based on the assumption of negligible coupling
between the longitudinal and lateral modes of motion. The method
proposed by Bramwell (1976) was used to perform the trim analysis.
The stability and control derivatives were calculated by obtaining the
trim settings from the trim analysis. These derivatives were then used
to solve the roots of the characteristic equations of the longitudinal and
lateral modes of motion. The stability of the helicopters were
investigated firstly by examining the stability derivatives and secondly
through root-loci analyses.
The most important results were the following:
• The root-loci analyses indicated that a helicopter without a
horizontal stabiliser suffered from instability of the phugoid
mode. It was also found that the short-period motion of these
helicopters was heavily damped. Fitting a horizontal stabiliser to
these helicopters caused the phugoid motion to become stable
even at low speeds. This was achieved at the cost of a reduction
in short-period motion damping.
• The periods of the lateral and longitudinal motions were smaller
than those found on full-scale helicopters. This was attributed to
the small mass and inertia properties of the model helicopters. An increase in speed is generally accompanied by an increase in
the stability of the helicopters. This could be attributed to the
effective operation of the tail surfaces at higher speeds.
• The axial climbing speed of a helicopter is influenced by the rotor
speed. A low rotor speed allows higher climbing velocities at a
given power setting. This was due to lower induced power losses
at low rotor speed, assuming that no blade stall occurs.
• The rotor speed does not influence the incremental amount of
power (M:,) required to achieve a certain climbing velocity, due
to the fact that the profile power losses are constant for a certain
rotor speed.
• The simplified horseshoe-vortex theory can be used to analyse
the downwash angle at the horizontal stabiliser if the helicopter
is in high-speed forward flight.